Lasers are recognized as controllable sources of radiation that is relatively monochromatic and coherent (i.e., has little divergence). Laser energy is applied in an ever-increasing number of areas in diverse fields such as telecommunications, data storage and retrieval, entertainment, research, and many others. In the area of medicine, lasers have proven useful in surgical and cosmetic procedures where a precise beam of high energy radiation causes localized heating and ultimately the destruction of unwanted tissues. Such tissues include, for example, subretinal scar tissue that forms in age-related macular degeneration (AMD) or the constituents of ectatic blood vessels that constitute vascular lesions.
The principle of selective photothermolysis underlies many conventional medical laser therapies to treat diverse dermatological problems such as leg veins, portwine stain birthmarks, and other ectatic vascular and pigmented lesions. The dermal and epidermal layers containing the targeted structures are exposed to laser energy having a wavelength that is preferentially or selectively absorbed in these structures. This leads to localized heating to a temperature (e.g., to about 70° C.) that denatures constituent proteins or disperses pigment particles. The fluence, or energy per unit area, used to accomplish this denaturation or dispersion is generally based on the amount required to achieve the desired targeted tissue temperature, before a significant portion of the absorbed laser energy is lost to diffusion. The fluence must, however, be limited to avoid denaturing tissues surrounding the targeted area.
Fluence, however, is not the only consideration governing the suitability of laser energy for particular applications. The pulse duration and pulse intensity, for example, can impact the degree to which laser energy diffuses into surrounding tissues during the pulse and/or causes undesired, localized vaporization. In terms of the pulse duration of the laser energy used, conventional approaches have focused on maintaining this value below the thermal relaxation time of the targeted structures, in order to achieve optimum heating. For the small vessels contained in portwine stain birthmarks, for example, thermal relaxation times and hence the corresponding pulse durations of the treating radiation are often on the order of hundreds of microseconds to several milliseconds.
The use of even shorter pulses, however, results in a change from photothermal to photomechanical processes. The latter mechanism is invoked by applying laser pulses having a duration that is below the acoustic transit time of a sound wave through targeted particles. This causes pressure to build up in the particles, in a manner analogous to the accumulation of heat within a target irradiated by laser pulses having a duration that is below the thermal relaxation time.
Photomechanical processes described above can provide commercially significant opportunities, particularly in the area of treating skin pigmentations including tattoos, portwine stains, and other birthmarks. The incidence of tattoos in the U.S. and other populations, for example, continues at a significant pace. Because tattoo pigment particles of about 1 micron in diameter or less may be cleared from the body via ordinary immune system processes, stable tattoos are likely composed of pigment particles having diameters on the order of 1-10 microns or more. As the speed of sound in many solid media is approximately 3000 meters/second, the acoustic transit time across such particles, and consequently the laser pulse duration required to achieve their photomechanical destruction, is as low as hundreds of picoseconds. The acoustic transit time of a sound wave in a particle is calculated by dividing the radius of the particle by the speed of sound in the particle.
In addition to such short pulse durations, high energy laser pulses are needed for significant disruption of tattoo pigment particles and other pigmentations. Required fluences of several joules per square centimeter and treatment spot sizes of a few millimeters in diameter translate to a desired laser output with several hundred millijoules (mJ) per pulse or more. Unfortunately, current systems capable of such short pulse duration and high energy output are too complex and/or expensive for practical use in the treatment or removal of tattoos and other pigmentations. These devices generally require two or more lasers and amplifier stages, together with multiple electro-optical and/or acousto-optic devices.
Sierra and Russell (SBIR Proposal to the NIH, submitted December 1993) disclose a device of reduced complexity, which demonstrated 100 millijoules of output. The device uses a single laser gain medium that is common to two resonators. A Pockels cell is used to sequentially select one or the other of the two resonators. Operation requires applying a bias voltage to the Pockels cell to establish a modelocked pulse along the first resonator, switching the Pockels cell bias voltage to amplify the pulse along a second, separate resonator, and then switching the Pockels cell bias again to extract the amplified pulse. The gain or lasing medium, two polarizers, a Pockels cell, an acousto-optical device, and two mirrors are included along the optical pathway of the first resonator. The lasing medium, polarizers, electro-optical device, and an additional mirror are included along the optical pathway of the second resonator.
While this apparatus is less complex than multiple laser systems, it nevertheless requires a large number of optical components (e.g., seven or more). In addition, the voltages applied and switched at the Pockels cell are equal to the halfwave bias voltage of the Pockels cell, typically in excess of 5,000 volts. These voltages must be switched in less than a few nanoseconds, placing a significant demand on the switching electronics. Also, because the system utilizes the separate operation of two resonators, it is possible due to component limitations for radiation from one resonator to leak or “spill over” into another. A consequence of this is the generation of undesirable or “parasitic” pulses, particularly in the resonator used for amplification, which supports a significantly lower threshold for laser oscillation. Finally, the use of an acousto-optic modulator to achieve modelocking may require the constant adjustment of resonator length, as such devices operate only at discrete resonant frequencies.
The simpler alexandrite and other Q-switched lasers currently employed in the treatment of dermatological pigmentations cannot reliably achieve tattoo pigment particle clearance in a matter of only a few treatments, despite claims to the contrary. Consequently, there is a need in the art for laser apparatuses of relatively low complexity, such that they are practical for tattoo pigment particle removal and the treatment of other pigmented lesions. Such apparatuses, however, must also be capable of emitting laser radiation with the short pulse duration required to invoke photomechanical processes. As discussed above, this requires pulse durations on the order of several hundred picoseconds, or the acoustic transit time across targeted pigment particles. Also characteristic of such a device is the capability of achieving an output energy of several hundred millijoules or more.